Optical filtering

ABSTRACT

An optical wave, guide transmission filter has a 3 dB single mode waveguide coupler that is provided on one side of its coupling region with a spectrally matched pair of retro-reflecting optical waveguide Bragg grating reflectors. The optical path distance of one of the Bragg reflectors from the coupling region is greater than the equivalent distance of the other Bragg reflector by an amount greater than half the coherence length of a signal having a spectral width matched with that of the Bragg reflectors.

This application is a division of U.S. patent application Ser. No.08/594,471 filed Jan. 31, 1996, now U.S. Pat. No. 5,647,037.

BACKGROUND OF THE INVENTION

This invention relates to the optical filtering of an optical signalcontaminated by noise, and in particular to the filtering of such asignal having a spectral bandwidth significantly smaller than thespectral bandwidth of a channel within which that signal is constrainedto lie.

Such a situation is liable to occur for instance in a wavelengthdivision multiplexed (WDM) system. In such a system the spectral widthof an individual channel is much wider than the actual spectral width ofthe signal that is being transmitted on that channel. This is aconsequence of there being a tolerance upon the specified emissionwavelength of optical sources, typically semiconductor lasers, employedfor transmission of the signal traffic. These specified tolerances haveto make allowance for such factors as ageing and the effects oftemperature. In the case of a typical 10 Gbit/s WDM system, the signalbandwidth is 0.08 nm, while the channel bandwidth is a few nm. Althoughthe signal itself may have a bandwidth of only 0.08 nm, it will normallybe accompanied by broader bandwidth noise. Such noise may emanate fromspontaneous emission from a laser source and, in the case of a systemincluding optical amplifiers, from spontaneous emission from theamplifiers. For multiplexing or demultiplexing purposes, a transmissionfilter will normally be required to have a bandwidth equal to thechannel bandwidth, in this instance typically 2 nm wide. Any attempt toattempt to attenuate the noise power extending over the spectral rangeof the channel will attenuate the signal by an equivalent amount, and sowill provide no improvement of signal to noise ratio.

SUMMARY OF THE INVENTION

The present invention is directed to filtering that can provide animprovement in signal to noise ratio.

According to the present invention there is provided a method offiltering a noise contaminated optical signal the spectral width ofwhich signal is small compared the spectral width of a channel withinwhich the signal is constrained to lie, in which method the noisecontaminated signal is divided into components which are caused topropagate different optical path distances before being recombined afterreflection in, or transmission through, spectrally matched spectrallyselective optical filter elements that are spectrally matched with thespectral width of the channel, wherein the difference in said opticalpath distances is great enough substantially to preclude coherentrecombination of noise power extending over the spectral range of thefilter elements while being small enough to provide substantiallycomplete coherent recombination of the signal power.

The invention also provides an optical waveguide transmission filterhaving a 3 dB single mode optical waveguide coupler provided, on oneside of its coupling region, with a spectrally matched pair ofretro-reflecting optical waveguide Bragg grating reflectors, one on eachof the two limbs of the 3 dB coupler, wherein the difference in opticalpath distance from the coupling region of the 3 dB coupler to each ofthe two Bragg reflectors is greater than half the coherence length of asignal having a spectral width matched with that of the Braggreflectors.

BRIEF DESCRIPTION OF THE DRAWINGS

There follows a description of an optical filter embodying the inventionin a preferred form. The description is prefaced with an explanation ofthe principles underlying the operation of the filter. The explanationand description refer to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a Michelson interferometer, and

FIG. 2 is a schematic diagram of the preferred embodiment of filter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring in the first instance to FIG. 1, the basic components of aMichelson interferometer comprise a 3 dB beam splitter 10 which dividesan input beam 11 into two equal amplitude components 12 and 13. Thesetwo components are subsequently recombined by the same beam splitter 10after each beam has been reflected in an associated reflector 14, 15. Itwill be observed that though the beam splitter 10 serves to combine thereflected components 12 and 13, it does not necessarily combine theminto a single combined beam. Generally it forms two combined outputbeams 16 and 17 whose relative power levels depend upon the differencein optical path distance travelled by the two component beams 12 and 13before they are recombined by the beam splitter 10. If this differenceis greater than the coherence length of the input beam 11, then there isno optical interference between the two reflected component beams.Accordingly the power in each reflected component beam 12 and 13 isdivided equally between output beams 16 and 17. If however thedifference in optical path distance travelled by the two component beams12 and 13 is significantly less than the coherence length of the inputbeam, then the division of the emergent power between output beams 16and 17 is dominated by interference effects. In particular, if thedifference is zero, or an integral number of wavelengths long, then thereflected component beams 12 and 13 will interfere with a phaserelationship that causes substantially all the output power to emerge byway of output beam 16. If the value of the optical path distance ischanged so as to alter the phase relationship by π, then substantiallyall the output power will emerge by way of output beam 17. Forintermediate values the power will be shared between the two outputbeams in a proportion determined by the phase relationship.

In FIG. 1 the 3 dB beam splitter 10 has been represented as a discretebulk optics component, but it can alternatively be constituted by anoptical waveguide format component. The reflectors 14 and 15 maysimilarly be constituted by optical waveguide reflectors, such asretro-reflecting Bragg grating reflectors. The optical waveguide format3 dB coupler and reflectors may be optical fibre waveguide components,or may form components of an integrated optics structure. Bragg gratingreflectors are spectrally selective, and so the device of FIG. 1constructed with an optical waveguide format 3 dB coupler 10 and Bragggrating reflectors 14 and 15 can be employed as a form of spectrallyselective optical filter. Such an arrangement of 3 dB coupler and Braggreflectors is for instance described in the specification of patentapplication Ser. No. 94308102.6, which relates to a construction thatautomatically ensures that the two gratings are optically equidistantfrom the coupling region of the 3 dB coupler. Such an arrangement isalso described in a paper by F Bilodeau et al entitled `High-Return-LossNarrowband All-Fiber Bandpass Bragg Transmission Filter`, IEEE PhotonicsTechnology Letters, Vol. 6, No 1, pp 80-2, which relates to aconstruction that includes a trimmer for adjusting the optical pathdistance of one of the gratings from the coupling region so as to beable to provide the requisite phase relationship between the lightreflected by that grating and the light reflected by the other gratingwhen they interfere in the coupling region of the 3 dB coupler.

Such a filter with zero optical path distance difference from thecoupling region to each of the Bragg grating reflectors can be used forselecting one of the channels of the WDM system to which previousreference has been made. This will require Bragg grating reflectors withspectral widths matched with that of the channel concerned but, aspreviously explained, any attenuation provided by this filter to reducenoise power extending evenly over this spectral band will attenuatesignal power to the same extent, and thus will not achieve any signal tonoise ratio improvement. However, with particular reference to FIG. 2,it will now be explained how such an improvement can be effected bymodifying the filter design so as to incorporate a significantdifference in optical path difference from the coupling region of the 3dB coupler to each of the Bragg grating filters, specifically adifference greater than half the coherence length of a signal having aspectral width matched with that of the Bragg reflectors.

Referring now to FIG. 2, an optical waveguide 3 dB coupler 20 has twosingle mode waveguide limbs 21 and 22 which are optically coupled in anoptical coupling region 23. On one side of this coupling region there isa spectrally matched pair of retro-reflecting Bragg grating reflectors24 and 25, one on each waveguide limb. The two Bragg reflectors are notequidistant from the coupling region 23: the difference in optical pathdistance (product of physical distance with effective refractive index)from the coupling region to each of the two Bragg reflectors is a valueof `d` units length. This value is greater than half the coherencelength of a signal having a spectral width matching that of the Braggreflectors.

The optical waveguide limbs may be constructed in integrated opticsformat or in optical fibre format. The Bragg gratings may be created inthe waveguides by irradiation with a fringe pattern of relatively highintensity ultra-violet light. Such a fringe pattern can be provided bytwo-beam interference effects, for instance as described in U.S. Pat.No. 4,275,110. An alternative way of providing such a fringe pattern canbe with the aid of a phase grating as for instance described in a paperby K O Hill et al entitled `Bragg gratings fabricated in monomodephotosensitive optical fiber by UV exposure through a phase mask`, Appl.Phys Lett., Vol. 2, No. 10, pp 1035-7.

It is easily seen that, if the optical path distance difference `d` werezero, then all the noise present within this 2 nm band will betransmitted by the filter along with the signal that has a bandwidth ofonly 0.08 nm. However, by making the optical path distance difference`d` a non-zero value, the light reflected back into the coupling region23 by Bragg reflector 25 will have had to travel a greater distance thanthat reflected by Bragg reflector 24, the difference in distanceamounting to `2d`. Clearly, if the optical path distance difference `d`is greater than half the coherence length of an optical transmissionhaving a bandwidth of 2 nm, then `2d` is greater than the full coherencelength. Accordingly, so far as the 2 nm bandwidth noise is concerned,the components reflected by the respective Bragg reflectors 24 and 25 donot interfere in the coupling region 23. Consequently only half of thepower of this noise is transmitted by the filter, the balance beingreflected by it. In contrast to this, the signal has a much longercoherence length on account of its much narrower bandwidth, and hence,provided that `d` is not very much longer than the coherence length ofthe 2 nm wide optical transmission, there is almost complete overlap andtherefore almost total interference between the two reflected portionsof the 0.08 nm bandwidth signal. The division of the reflected signalpower between the two limbs depends, as explained previously, upon therelative phase of the two reflected portions on their return passagethrough the coupling region. This relative phase is a function of thewavelengths of the signal and of the magnitude of the optical pathdistance difference `d`, and therefore either of these parameters can inprinciple be adjusted to provide the requisite relative phase for allthe interfering light to be coupled by the filter from one limb of the 3bB coupler into the other (i.e. for the filter to act as a transmissiontype filter). For the particular wavelength division multiplexed systemapplication referred to above, the wavelength of the signal is variableover a range that will compass many cycles of relative phase angle, andso in this instance the adjustment necessary to ensure the requisiterelative phase will normally be effected by way of adjustment of theoptical path distance difference `d` by means of a variable delayelement 26 inserted in one of the limbs 21, 22 between the couplingregion 23 of the 3 dB coupler 20 and the associated Bragg reflector 24,25.

I claim:
 1. A method of filtering a noise contaminated optical signalthe spectral width of which signal is small compared with the spectralwidth of a channel within which the signal is constrained to lie, inwhich method the noise contaminated signal is divided into componentswhich are caused to propagate different optical path distances beforebeing recombined after reflection in, or transmission through,spectrally matched spectrally selective optical filter elements that arespectrally matched with the spectral width of the channel, wherein thedifference in said optical path, distances is great enough substantiallyto preclude coherent recombination of noise power extending over thespectral range of the filter elements while being small enough toprovide substantially complete coherent recombination of the signalpower.
 2. A method as claimed in claim 1, wherein the division of thenoise contaminated signal is effected by means of a 3 dB opticalwaveguide coupler.
 3. A method as claimed in claim 2, wherein therecombination of the noise contaminated signal is effected by means of a3 dB optical waveguide coupler.
 4. A method as claimed in claim 3,wherein the recombination is effected after reflection in Bragg gratingspectrally selectively reflecting optical filter elements.